It has long been recognized that it would be highly desirable to have a reliable means of determining the capacity of lead-acid batteries. Over the years, a considerable amount of effort has been directed to providing an economical, yet accurate, means of indicating the capacity of lead-acid batteries.
It has been recognized, that, in lead-acid batteries, the average concentration of the bulk electrolyte is proportional to the capacity of such a battery. Sulfuric acid is thus involved in the following electrochemical reaction in lead-acid batteries: EQU Pb+PbO.sub.2 +2H.sub.2 SO.sub.4 .revreaction.2PbSO.sub.4 +2H.sub.2 O
Accordingly, as may be seen from the above-identified electrochemical reaction, the specific gravity of the electrolyte is raised and lowered as the battery is charged and discharged, respectively.
While simple in principle, the measurement of the average acid concentration in lead-acid batteries is complicated by the size of batteries, acid stratification, and the corrosive battery environment itself. For example, acid concentration gradients or stratification occurs when lead-acid batteries are charged or discharged. Accordingly, measurements can easily have more than 30% error if just the top part of the sulfuric acid is used in determining the specific gravity. Indeed, under certain conditions, an error of the magnitude of anywhere from 50 to 100% can result.
Ideally, perhaps, the open circuit voltage could be used as an indication of the specific gravity of the electrolyte. Unfortunately, however, the use of the open circuit voltage of a lead-acid battery does not give an accurate indication of the average bulk acid concentration unless the acid is destratified by vigorously gassing or by the use of other mechanical means, and the PbO.sub.2 plates are also stabilized for a long period of time.
U.S. Pat. No. 3,659,193 to Pitsch et al. measures the concentration of an electrolyte by utilizing two electrodes immersed in the electrolyte. U.S. Pat. No. 4,045,721 to Swain discloses, what is termed, a proportional electric comparator of strong electrolytes. A sensor is made by placing a sensor electrode in a sensed electrolyte, and a reference electrode in a stable electrolyte, and electrically connecting the electrolytes with a capillary-joining electrolyte. This sensor is ordinarily constructed so that its electrical resistance is proportional to the resistivity of the joining electrolyte.
U.S. Pat. No. 4,129,824 to Howes discloses an electronic hydrometer for monitoring the specific gravity of a solution. This hydrometer includes a probe having spaced apart electrodes that are adapted to be inserted into the solution to be monitored and is provided with A.C. resistance electrically connected in series with an electrode of the probe to form, with the probe, a voltage divider network.
U.S. Pat. No. 4,689,571 to Yonezu et al. describes, by way of background, various types of specific gravity sensors that have been developed which provide an output that changes according to changes in the specific gravity of the electrolyte, as well as what are considered to be the disadvantages of those various types. Yonezu et al. disclose a lead electrode and a lead dioxide electrode which are immersed in the electrolyte, and the potential between those electrodes is converted to data indicative of the specific gravity of the electrolyte. A method for stabilizing the potential of the electrodes used is likewise disclosed.
Despite all of the substantial prior efforts in this field, there still exists the need for a reliable and efficient means for determining the capacity of a lead-acid battery. Accordingly, a principal object of the present invention is to provide a lead-acid battery having a reliable and efficient capacity indicator. A more specific object provides a battery including an indicator which can reliably and rapidly determine the average specific gravity of the electrolyte in the battery.
Another object of this invention provides a lead-acid battery having a capacity indicator whose accuracy is essentially independent of the extent of acid stratification within the battery.
Yet another object lies in the provision of a capacity indicator that can be readily and economically incorporated into existing commercial lead-acid battery configurations.
A still further object of the present invention is to provide a capacity indicator that will be operable over the expected life of a lead-acid battery.
Another and more specific object of the present invention is to provide a capacity indicator that may be utilized with a sealed lead-acid battery.
Yet another object of this invention provides a capacity indicator allowing the dynamic measurement of the capacity of a lead-acid battery.
An additional object of this invention lies in the provision of a direct electrical signal output to an electrical instrument for controlling the charging and/or discharging of lead-acid batteries.
Other objects and advantages of the present invention will become apparent as the following description proceeds.